The effect of insulin on postprandial lipoprotein metabolism will be studied in normal, obese and diabetic subjects. Each subject will be studied on 3 occasions following a lipid meal: on occasion 1, endogenous responses to a fatty meal will be measured; on occasion 2, an exogenous infusion of insulin will be given while blood glucose is kept constant by infusion; on occasion 3, the response to a mixed lipid and carbohydrate meal will be measured.